The present invention relates to apparatus and methods for reducing NO.sub.x emissions from a gas turbine and particularly relates to apparatus and methods for reducing NO.sub.x in a gas catalytic combustion system with preburner over the entire operating range of the gas turbine.
As set forth in my prior U.S Pat. No. 4,845,952, the objectives of many manufacturers of gas turbines include operating at high efficiency without producing undesirable air-polluting emissions. Conventional fuels normally used in gas turbines, when burned, typically produce oxides of nitrogen, carbon monoxide and unburned hydrocarbons.
NO.sub.x compounds are produced by the reaction of nitrogen in the air at the elevated temperatures conventionally found in combustors of gas turbines. NO.sub.x formation can be reduced by reducing maximum flame temperature in the combustor, for example, by introduction of steam. However, penalties to thermodynamic efficiency and increased capital costs are incurred. It is known to use a combustion catalyst in the reaction zone of a gas turbine combustion system to promote complete combustion of lean pre-mixed fuel and air to minimize the level of air-polluting emissions. Catalytic combustion occurs at a relatively low temperature insufficient to generate NO.sub.x from nitrogen and oxygen reactions which occur at higher temperatures. It will be appreciated, however, that when combustor inlet air temperature and temperature rise across the combustion system are too low to support catalytic combustion, a diffusion flame preburner may be used to obtain catalytic reactor ignition. That is, catalytic combustion alone cannot be used over the entire operating range of the gas turbine because the inlet air temperature and temperature rise across the combustion system are too low to initiate and sustain pre-mixed catalytic combustion during gas turbine ignition, acceleration and operating at the low end of the gas turbine load range.
When using a diffusion flame preburner, however, significant amounts of NO.sub.x emissions are generated. Prior catalytic combustion system designs do not include methods for reducing the preburner NO.sub.x emission. Consequently, while low NO.sub.x emissions are obtained over the mid-operating range of the gas turbine combustion system, prior catalytic combustion system designs do not include any method of reducing NO.sub.x emissions from the preburner. Accordingly, a purpose of the present invention is to provide a catalytic combustion system and method of preburner NO.sub.x abatement such that the catalytic combustion system may operate with extremely low NO.sub.x emissions over the entire operating range of the gas turbine.
According to the present invention, there is provided a catalytic combustor with a diffusion flame preburner for a gas turbine system for minimizing NO.sub.x emission throughout the operating range of the turbine. Three different operating modes for this combustion system are provided over the load range of the gas turbine. The first operating mode is a low-load operating condition of the gas turbine where only preburner combustion occurs with chemical/catalytic NO.sub.x removal, i.e., deNO.sub.x. For example, hydrocarbon fuel may be supplied to a preburner start-up fuel nozzle and air may be directed to a preburner combustion zone. An electrical ignition device, such as a spark or glow plug, ignites the fuel/air mixture in the preburner combustion zone with the flame being stabilized by vortex recirculation generated by swirl vanes in the start-up fuel nozzle. Significant amounts of thermal NO.sub.x are generated by this diffusion flame reaction within the preburner combustion liner. To reduce this NO.sub.x to molecular nitrogen and water vapor, a chemical reactant, such as ammonia, urea, isocyanic acid or the like may be injected through the primary injector for the catalytic combustion section (used during mid and high-load operating ranges) into the preburner products of combustion. Mixing may be promoted by the infusion of nitrogen with the chemical reactant. The chemical reactant may also include enhancers to accelerate the rate of reaction with NO.sub.x from the diffusion flame preburner. The chemical reaction may occur within the catalytic reactor assembly liner and the catalytic reactor bed of the catalytic combustion zone, including the catalyst, to accelerate the deNO.sub.x chemical reactions.
In a second operating mode characterized as a mid-load operating range for the gas turbine, catalytic combustion occurs. To achieve this, fuel is supplied by the primary injector and mixed with the preburner products of combustion. This mixture enters the catalytic reactor bed which contains a combustion catalyst, for example, palladium. This mixture of fuel and preburner products of combustion ignites in the presence of the combustion catalyst at preburner discharge temperature. Once the combustion reaction has been initiated, the preburner may be shut down, with the reaction being sustained at compressor discharge air temperature. By introducing a lean fuel/air mixture into the catalytic reactor bed, combustion reaction temperature is maintained too low to produce thermal NO.sub.x. The hydrocarbon fuel oxidation reactions go to completion in the reaction zone within the main combustion liner. Thus, the NO.sub.x emissions during low and mid-range operating conditions are substantially eliminated or minimized to ultra-low emissions.
At high load operating conditions for the gas turbine, a combination of catalytic and pre-mixed combustion is provided. The catalytic reactor operates in the same manner previously described as in the second operating mode, i.e., mid-range catalytic combustion. A secondary injector, however, is provided for mixing hydrocarbon fuel with compressor discharge air. This fuel/air mixture enters the reaction zone within the main combustion liner and is ignited by the hot products of combustion exiting the catalytic reactor bed. Because this fuel/air mixture is lean, combustion reaction temperature is likewise too low to produce thermal NO.sub.x. In this manner, NO.sub.x emissions are substantially minimized or eliminated throughout the entire operating range of the gas turbine.
In a preferred embodiment according to the present invention, there is provided a method of operating a gas turbine catalytic combustion system having a preburner section and a catalytic combustion section to minimize or eliminate NO.sub.x emissions comprising the steps of combusting a fuel/air mixture in the preburner section, reducing the NO.sub.x resulting from the combustion of the fuel/air mixture in the preburner section, operating the preburner section to obtain catalytic reaction ignition and, upon ignition, operating the catalytic combustion section at a combustion temperature too low to produce NO.sub.x whereby NO.sub.x emissions from the gas turbine operation are substantially minimized or eliminated.
In a further preferred embodiment according to the present invention, there is provided a method of operating a gas turbine catalytic combustion system at low-load and mid-load ranges of gas turbine operation wherein the combustion system has a preburner section and a catalytic combustion section, comprising the steps of, at low-load operation, supplying a fuel/air mixture in the preburner section for combustion, reducing the NO.sub.x resulting from the combustion of the fuel/air mixture in the preburner section, operating the preburner section to obtain catalytic reactor ignition in the catalytic combustion section and, upon ignition and at mid-load range, operating the catalytic combustion section with a lean fuel/air mixture such that the combustion reaction temperature is too low to produce thermal NO.sub.x whereby NO.sub.x emissions from gas turbine operation at low and mid-load ranges of operation are substantially minimized.
In a still further preferred embodiment according to the present invention, there is provided a gas turbine catalytic combustion system with low NO.sub.x emissions comprising a preburned section, means for introducing fuel and air into the preburner section, an igniter in the preburner section for combusting the fuel/air mixture and means for reducing the NO.sub.x in the products of combustion of the preburner section. A catalytic combustion section is provided having a catalytic reactor bed having a catalyst and a reaction zone. Means are provided for introducing a lean mixture of fuel and air into the catalytic combustion bed with catalytic combustion occurring at least initially from ignition by the preburner products of combustion in the presence of the catalyst in the bed. Means are also provided for mixing compressor discharge air and fuel and supplying the mixture to the reaction zone of the combustion section for ignition by the hot products of combustion exiting the catalytic reactor bed.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for operating a catalytic combustion gas turbine system which achieves ultra-low NO.sub.x emissions over the entire operating range of the gas turbine.